(define (make-leaf symbol weight)
    (list `leaf symbol weight)
)
(define (leaf? object)
    (eq? (car object) `leaf)
)
(define (symbol-leaf x) (cadr x))
(define (weight-leaf x) (caddr x))

(define (make-code-tree left right)
    (list 
        left
        right
        (append 
            (symbols left)
            (symbols right)
        )
        (+ (weight left) (weight right))
    )
)

(define (left-branch tree) (car tree))
(define (right-branch tree) (cadr tree))
(define (symbols tree)
    (if (leaf? tree)
        (list (symbol-leaf tree))
        (caddr tree)
    )
)
(define (weight tree)
    (if (leaf? tree)
        (weight-leaf tree)
        (cadddr tree)
    )
)

(define (decode bits tree)
    (define (decode-1 bits current-branch)
        (if (null? bits)
            `()
            (let 
                (
                    (next-branch (choose-branch (car bits) current-branch))
                )
                (if (leaf? next-branch)
                    (cons 
                        (symbol-leaf next-branch)
                        (decode-1 (cdr bits) tree)
                    )
                    (decode-1 (cdr bits) next-branch)
                )
            )
        )
    )
    (decode-1 bits tree)
)
(define (choose-branch bit branch)
    (cond 
        ((= bit 0) (left-branch branch))
        ((= bit 1) (right-branch branch))
        (else (error "bad bit -- CHOOSE-BRANCH" bit))
    )
)

(define sample-tree
    (make-code-tree
        (make-leaf `A 4)
        (make-code-tree 
            (make-leaf `B 2)
            (make-code-tree
                (make-leaf `D 1)
                (make-leaf `C 1)
            )
        )
    )
)
(define sample-message `(0 1 1 0 0 1 0 1 0 1 1 1 0))
(display (decode sample-message sample-tree))
(exit)